1. Field of the Invention
This invention relates to a process for the production of curved panes intended notably for automobile vehicles, according to which each sheet of glass is, in a first period, heated above its deformation temperature, conducted into a curving station situated in the continuation of the furnace and in which the high temperature of the glass is maintained and then, after curving, is transferred into a cooling station, for example a thermal quenching station.
2. Description of the Related Art
For producing automobile panes, notably fixed side panes, side window panes, rear panes or windscreens, it is necessary to shape a flat sheet of glass which is cut to the dimensions of the pane, and then to carry out lamination or, more frequently, tempering or toughening of the curved pane in order to give greater safety to the passengers in the case of fracture of the pane.
Many methods of curving, termed horizontal methods, are known, in which the glass sheet is heated above its softening point (500.degree.-700.degree. C.) in an electric furnace, which it passes through conveyed, for example, by a bed of motorized rollers. The heating furnace is followed by a curving station, in which the glass is shaped by specific curving tools and, for example, in which the glass sheet is raised above the conveyor, and applied against a curving mold, and then finally collected by a carriage carrying a ring profiled in a manner corresponding to the final form of the curved pane. Thereafter, the carriage removes the glass sheet, for example towards a quenching or tempering device or other cooling device.
In other cases, reserved almost exclusively for the manufacture of laminated panes, the pair or pairs of glass sheets is deposited, at the entry of the furnace, onto a curving mold, on which it is directly heated for a curving termed curving by collapse.
Without discussing in greater detail the technique of curving used, it should be noted that the greater the accuracy required, and the more complex the shape, the slower the curving operation will be. To operate too rapidly leads to a risk of temporarily creating stresses higher than the strength of the glass and therefore of fracturing the glass. Furthermore, when the cooling step consists of a thermal quenching, for example by blowing on cold air, it is essential for the pane to still be as hot as possible when it enters the tempering station.
For these reasons, the Applicant has chosen, in the present case, to investigate only those shaping methods according to which the temperature of the glass remains virtually constant throughout the curving operation. Constant temperature does not signify total absence of heat exchange with the glass sheet, and moreover the speeds of passage of the glass sheets do not systemically guarantee perfect thermal equilibrium (and this in any case may not be desired, since local overheating promotes folding at these points, for example). On the other hand, the heat exchanges remain of extremely small amplitude, in that it is chosen to carry out the molding in a hot enclosure, the temperature of which is close to that of the glass and not in the open air, where the glass initially at about 650.degree. C. would undergo a very considerable cooling, which would be detrimental to good shaping quality and in particular to the quality of quenching.
From the foregoing considerations it is clear that it is absolutely necessary to avoid abrupt and uncontrolled variations in temperature in the curving station. Moreover, although it is relatively easy thermally to insulate a closed enclosure, it is quite otherwise in the case of a curving station which, of necessity, comprises two entries-exits (for the introduction and removal of the glass sheets and/or the frames mounted on a carriage transporting the curved glass). Lateral leakages occur via the conveyor which removes the glass sheets from the furnace. The conveyor is most commonly composed of rollers, the pinions of which engage on chains arranged outside the hot enclosure in order to simplify the adjustments and, in particular, to obtain reasonably long working lives for the drive motors for these rollers, for which it is well known that it is virtually impossible to operate them continuously and in a satisfactory manner at a temperature in the vicinity of 650.degree. C.
At the inlet side, that is to say at the furnace side, there is no possibility of cold air entering, in view of the fact that the curving station is at the same temperature as the furnace, of which it constitutes, in the limit, the extreme downstream element. The lateral leakages may, furthermore, be prevented to a great extent by appropriate filling or stopping-up. On the other hand, and to the extent to which the production line is situated in a building not subject to flows of air, the feed of cold air which takes place is virtually constant and can therefore be compensated by appropriate heating. In contrast, it is quite otherwise in the case of the outlet from the curving station.
In fact, the opening must be sufficiently wide to enable a curved sheet of glass to pass through, or in certain cases with a height of more than 20 centimeters and a width of approximately 1 m 50 cm if a rear pane is considered. Unless an extraordinarily high energy consumption can be accepted, it is entirely unacceptable to leave an aperture of such dimensions permanently open. And even if it were assumed that the energy cost could be accepted, the special problem of the entry of cold air consequent upon the blowing of cold air for quenching would not be solved, so that it is practically impossible to create relatively isothermic conditions.
In reality, all the "hot" curving stations are equipped with an outlet door with means for rapid opening and closing. Nevertheless, each time this door is opened, a very considerable amount of cold air enters. In a plant with a high throughput rate, the instant at which the door opens to permit the exit of a glass sheet, the shaping of which has just been completed, may correspond to the quenching of the preceding glass sheet and the entry into the curving station of the succeeding glass sheet. The flow of cold air will therefore lick the rollers and, in, particular, will cool the succeeding glass sheet, which evidently is very harmful to the latter.
To overcome this, it is known to line the door of the curving station with a hot air curtain, generated by burners situated in the curving station. A descending curtain of hot air has the disadvantage of pushing back the cold air penetrating into the station towards the conveyor and therefore towards the succeeding glass sheets, whereas an ascending curtain of hot air must be emitted at a pressure sufficiently low so as not to cause the lifting of the glass sheet which is passing through the hot air curtain. This systematic limitation to the permitted pressure does not allow complete compensation of the flow of cold air, so that it is necessary to provide precise synchronization of the movements of the different glass sheets, in order that the arrival of the glass sheet shall be sufficiently late for homogenization of the temperature of the curving station to take place between two glass sheets.